1. Field of the Invention
The present invention relates to a process for manufacturing silane, and more particularly it relates to an improved process for manufacturing silane by a disproportionation reaction of a trialkoxysilane.
2. Description of the Prior Art
Silane is useful as a raw material of grown epitaxial silicon film, amorphous silicon for solar cells and light-sensitive drums of electronic duplicating machines, and polycrystalline silicon for semiconductors.
Heretofore, silane has been manufactured by the following known methods: (1) A method of reacting magnesium silicide with an ammonium salt in liquid ammonia (e.g., Japanese Patent Publication No. 38-19951); (2) a method of reducing chlorosilane with a metallic hydride such as lithium hydride (e.g., Japanese Patent Publication Nos. 39-3660 and 59-1211); (3) a method of disproportioning chlorosilane in the presence of a catalyst such as a compound containing an .alpha.-oxo-amine group (e.g., Japanese Patent Laid-open Application No. 59-54617); or (4) a method of disproportioning a trialkoxysilane in a liquid phase in the presence of a catalyst such as sodium ethoxide, magnesium acetylacetonate, lithium chloride or hexamethylphosphoric triamide (e.g., Japanese Patent Publication Nos. 51-20440 and 60-4195).
Of these conventional manufacturing methods, the first method has drawbacks that an especial compound such as magnesium silicide must be employed, and that liquid ammonia is used and, therefore, facilities for maintaining high pressure and low temperature are necessary, which requires large investments. In addition, the first method involves the disposal of a sludge of magnesium salts containing ammonia.
In the second method mentioned above, a dangerous compound such as the metallic hydride which is to be handled with much attention must be employed, and in addition, by-products such as chlorine and lithium chloride are formed in quantities. In this case, in order to recover and reuse these by-products, a complex recycling system must be developed. Further, in common with the third method mentioned above, corrosive chlorosilane is employed as a starting material, and, therefore, there is the disadvantage that measures must be taken against the corrosion of the apparatus used.
The fourth method mentioned above is based on the fact that a trialkoxysilane can be disproportioned more easily than trichlorosilane and the fact that the disproportionation reaction makes progress even at room temperature in the presence of a suitable catalyst in order to obtain a disproportioned product such as silane. Thus, in the fourth method, the disproportionation reaction is performed under moderate heating conditions in a liquid phase reaction. This method seems to be ideal, but in fact it is not so. In the case that a dangerous compound such as silane is manufactured on an industrial scale, it is essential that the reaction can be easily controlled. However, this method does not satisfy such a requirement. That is, in the method of the disproportionation reaction of a trialkoxysilane by a liquid phase reaction, the prompt termination of the reaction cannot be made in the middle of the reaction, when needed. The reasons are that the catalyst is dissolved or suspended in the liquid reaction system, or the solid catalyst is always in contact with the material, and that the reaction heat is elevated, even if heating of the reactor is stopped and the system is cooled to room temperature. As understood from the foregoing, the fourth conventional method has the serious drawback that the reaction cannot be stopped promptly.